The Elastic Mechanical Properties of Supported Thin Polymer Films.

Abstract

This dissertation provides new and comprehensive insights into the nanoscale elastic mechanical response of supported polymer films. It is shown, using an atomic force microscopy nanoindentation technique, that thin polymer films supported by stiff (non-compliant) substrates exhibit effective elastic moduli E that increase with decreasing film thickness, h, for films thinner than a threshold film thickness ht. The magnitude of ht is a function of the polymer and its magnitude is typically in the range of a few hundred nanometers. A diverse range of polymer systems was investigated: (1) linear-chain polymers, (2) a miscible polymer/polymer blend system, and (3) star-shaped polymers. For the case of linear-chain polymers, it is shown that indentation-induced stress field could be two orders of magnitude larger than the actual indentation depth, and the degree of enhancement of the effective modulus differs for different polymers. While ht for polystyrene (PS) and poly(methyl methacrylate) (PMMA) films were comparable, ht was smaller for polycarbonate (PC) films: ht(PS) ~ ht(PMMA) ~ 450 nm > ht(PC) ~ 300 nm. In contrast to the current understanding of the field, it was shown that the elastic mechanical response of polymer films could not be fully understood in terms of the macroscopic modulus and Poisson’s ratio of individual polymers and polymer/substrate interfacial interactions. We showed, for the first time, that this behavior is correlated with the local vibrational force constants (i.e. local stiffness), typically measured using incoherent neutron scattering, of the polymer films. The elastic mechanical response of a miscible polymer/polymer blend was also rationalized in terms of the local elastic behavior of the blend, at different compositions, determined from incoherent neutron scattering measurements. Finally, for the case of star-shaped PS molecules, the response was virtually identical to that of linear chain PS. However, ht for a short arm star-shaped PS, with f = 64 arms, was nearly 50% larger. This is associated with the fact that the structure of the molecule is different; the molecules formed an ordered structure similar that that of particles or colloids.